Transducers are defined as devices that convert energy from one form to another. Non-destructive testing (NDT) often uses special transducers to interrogate the internal structure and features of materials or devices under test (DUT) that can not be directly observed by ordinary means. In many instances the structures or features embedded in the materials are undesirable and are considered flaws or damage. In other instances the internal structures and features are an expected, allowable or necessary component of the material. In either case, obtaining accurate information about the nature and character of these internal structures and features is a fundamental need during the inspection, qualification and diagnosis of these materials. One common transducer in NDT applications is the ultrasound transducer (UT).
In the case of a UT, sound energy is converted to electrical energy or electrical energy is converted to sound energy. In many cases this energy conversion is done through the piezoelectric effect, where an electrical field or potential in a piezoelectric material induces a mechanical strain generating an acoustic field, or conversely, an acoustic field impinging on the piezoelectric material induces an electric field or potential. These piezoelectric materials normally have complex electrical impedances which require special methods to integrate them into useful transducers.
Ultrasound is defined as sound whose frequency is above the threshold of hearing (that is, greater than 20 kHz). For practical purposes, signals greater than 100 kHz are typically used. Frequency, along with excitation time width, defines the spatial resolution of the interrogating sound field. A short, high frequency pulse will enable small anomalies and defects in material to be detected. However, with an increase in frequency comes an increase in the attenuation through scattering and absorption of the sound energy. This limits the effective depth that can be probed without too great a loss in signal energy. Consequently, there is an optimal frequency range that trades resolution with received signal response. For ultrasound NDT, a frequency range between 2 and 20 MHz is optimal, with the lower range being most common.
A conventional NDT system configuration is shown in FIG. 1. In this figure the ultrasound system B is connected to a UT C via a long coaxial cable D. UT C includes piezoelectric material BP and an impedance matching and tuning network BI which operate and function in known manners. The ultrasound system produces a high voltage excitation signal which propagates down the coaxial cable, is applied to the piezoelectric element (which includes the piezoelectric material and optional tuning components) and generates an acoustic signal from the front of the UT. Received acoustic signals are sensed by the UT and converted to electrical signals, which are sent back down the coaxial cable to the ultrasound system for analysis. These received signals are generated due to an acoustic impedance mismatch in the material being interrogated. When the acoustic wave in the material impinges upon a change in material property (a change in either density or speed of sound which gives rise to a change in acoustic impedance), part of the acoustic wave is reflected, and another part is transmitted through. This reflected wave is what is received by the transducer. The location of either the far wall of the material or an internal crack or void is measured by the transit time for the wave to travel forward and then be reflected back. Corrosion shows up as a change in distance or transit time from the transducer to the far wall as compared with un-corroded sections.
A useful measure of the ability of a system to convey information from a transducer to a measurement or detection unit is the Signal-to-Noise Ratio (SNR). This parameter relates the signal, in whatever dimensions and units are appropriate, to the noise that is present and that would be detected simultaneously with the signal. High SNR values indicate that the signal stands out clearly against the background noise, whereas low SNR values indicate that the signal is being obscured by the noise.
High noise environments, long interconnections, lossy matching networks and low signal levels generally combine to degrade the SNR and confound the detection of small, deep or poorly defined structures or features in the material under interrogation.
Single crystal transducers limit the interrogation of material to the volume under the crystal. Consequently, to inspect more volume of the DUT, the transducer has to be moved. Movement of the transducer by the operator introduces the potential of operator error. Automated Ultrasound Testing systems (AUT) use robotics to move the transducer to remove the potential of operator error. However, these systems can be costly and are time consuming to set up.
Recently, NDT ultrasound systems have incorporated linear array transducers. A linear array transducer consists of a line of single rectangular piezoelectric elements closely spaced side by side. The height of each element is typically 1.5 times the width, and the inter-element spacing is on the order of half a wavelength. A typical linear array transducer will have 16, 64 or 128 such elements in a row. As known to those trained in the art, the geometry of the elements defines the electro-acoustic and geometric response of the transducer array. The thickness and type of piezoelectric material, along with the material loading the back and front sides of the piezoelectric material defines the frequency of operation of the transducer. The size and spacing of the elements defines the geometric response of the transducer as it is used in either single element, or multiple element operation. The ultrasound system or phased array system can pulse and sense single elements in a raster scan (see FIG. 2a) with the pulses to the various elements spaced relatively far apart in time from the pulses to other elements so that the acoustic waves from various elements do not superimpose.
Alternately, the elements can be pulsed either simultaneously or sequentially, introducing various delays between elements. FIG. 2b shows an example where the array elements are pulsed sequentially with small delays between the pulses to the various elements. This effectively steers the acoustic beam. By appropriate timing and control of the various elements in the array (establishing each as a “receive only”, “transmit only” or “receive-transmit” combination as described elsewhere in this application) in conjunction with the overall timing organization of the high voltage and receiver gating, phased array operation can be achieved on both transmitted and received acoustic signals.
A linear array system effectively increases the volume of material that can be inspected by sweeping the beam throughout a range of angles in one dimension or stepping the active elements along the direction of the array without the need to move the transducer, thereby reducing the potential of operator errors. The transducer still must be moved in the direction perpendicular to the angle of beam steering to gather more volume information from the DUT in two dimensions. In this configuration, an encoder is used to measure the amount of motion, so that as the transducer is moved, the area under it is appropriately mapped by the phased array system. Phased array systems are typically 5 to 10 times more costly than a single element system, and the transducers are more expensive with more complex connectors and cabling due to the increased number of wires required to access all the elements.
Two-dimensional transducer array systems are also available and provide the greatest volume of inspection without the need to move the transducer. However, the complexity and cost of these systems have made their use prohibitive in NDT. They are primarily used in certain medical applications, providing three-dimensional imaging. As with the linear array, the 2D array has an even more complex connector and cabling systems due to the increased number of elements. In the linear or 2D array, although a common, large area front electrode is typically used, the back electrode of each cell is typically tied directly to a wire, so that the number of wires leaving the transducer are equal to the number of elements plus one for the reference wire. Transducer size is also limited when interrogating curved surfaces because the transducers are inflexible. As a result they are unable to make contact over large areas of curved surfaces.
There is a need for a two-dimensional array transducer system that can provide a large volume of interrogation without the need for an operator to move the transducer. There is also a need to have this transducer flexible so that it bends over a curved surface. There is also a need to integrate the excitation and receive functions of the ultrasound system to reduce the need for extensive cabling. The objective of this invention is to achieve all of these goals while reducing or avoiding the undesirable effects identified herein.